1. Field of the Invention
This invention relates to wheel slip control safety timers that can suspend a wheel slip control operation.
2. Description of Related Art
It is well known in the brake control art that modern train brake control systems typically use a central controller unit to control the brakes of the rail vehicles that comprise the train. A train operator located in the lead cab manipulates the brake handles or like devices of the train to apply and release the brakes of the trucks or bogies on each rail vehicle as desired. The inputs from the brake handles are typically processed by a cab control unit and passed to the central controller unit. In response to these and other inputs, the central controller unit issues a brake command signal along a train line to each of the rail vehicles in the form of either a pneumatic signal or an electrical signal or even both. Brake equipment on each of the rail vehicles applies or releases the brakes according to the dictates of the particular brake command signal received. The central controller unit sends the brake command signal to each of the rail vehicles along either a pneumatic train line or an electrical train line.
Depending on the type of train being considered, the brake equipment on each rail vehicle may include either exclusively pneumatic equipment or a combination of electrical and pneumatic (i.e., electro-pneumatic) equipment. Each rail vehicle typically includes a local control system whose construction and operation are generally well known in the brake control art. The local control system on each rail vehicle receives the brake command signal and various other signals in response to which it directly controls electro-pneumatic equipment according to principles well known in the brake control art. Specifically, the local control system generates the electrical signals which open or close the various valves which supply pressure to or vent pressure from the brake cylinders. The brakes on each rail vehicle apply and release accordingly.
The prior art brake control systems typically provide service braking, emergency braking, and wheel slip control functions. The local control system includes a microprocessor-based device which performs service brake control, limited emergency brake control, and wheel slip control. Associated with the brakes on each bogie is a Brake Cylinder Control Unit (BCCU) which manipulates brake cylinder pressure based on pneumatic and/or electrical inputs from the local control unit. It provides service and emergency brake cylinder pressure control as well as wheel slip brake cylinder pressure modulation. The BCCU has integrated magnet valves and transducers which are monitored and controlled by the local control unit.
A common element of the BCCU is a wheel slip valve which functions to adjust the brake cylinder air pressure available to that which adhesion between wheel and rail will support. Typically, an external sensing device transmits electrical signals to the coils of magnet valves controlling pilot pressure to lap (maintain) and release valves. When the release coil is energized, the supply of air to the brake cylinders is cut off and the air from the brake cylinders is vented to atmosphere. When the braking effort has decreased by a sufficient amount, the external wheel slip sensing device serves to effect a lap or maintaining of brake cylinder pressure by energizing the lap coil.
Prior to this invention, the reduction of brake control pressure for wheel slip conditions was either inhibited or monitored by a hardware electronic safety timer. Safety timers for use with wheel slip control are known in the art, for example, as disclosed in U. S. Pat. No. 6,698,848. Depending on the configuration of the service brake components and wheel slip components, improvements in the existing safety timers are desirable.